This invention relates to a switched-mode power supply, comprising:
a transformer having a primary winding and at least an auxiliary winding; PA1 a controllable switch connected in series with the primary winding so as to receive an input voltage; PA1 a control circuit having an output coupled to a control input of the controllable switch to supply control signals to the controllable switch for turning on and turning off the controllable switch, and having a control input for receiving a control voltage for powering the control circuit and for controlling the timing of the turn-on and turn-off of the controllable switch; and PA1 a rectifier and a smoothing capacitor connected in series to the auxiliary winding to generate the control voltage. PA1 a charge pump coupled to the auxiliary winding to convert an alternating voltage across the auxiliary winding into a direct voltage across the smoothing capacitor.
Such a switched-mode power supply is known from the specifications of a switcher IC type TOP210 published July 1996 and which is commercially available from Power Integrations, Inc. In said switcher IC the controllable switch is a MOSFET switching transistor. The control circuit inter alia includes a pulse width controller for varying the ratio between the off period and the on period of the switching transistor. The duty cycle of the pulse width is determined by the amount of current which flows into the control input. The voltage on the control input is maintained constant by special means in the control circuit. This constant voltage also serves as the supply voltage for the control circuit. Since the smoothing capacitor is connected to the control input, the voltage on the smoothing capacitor in normal operation is equal to the constant voltage. As a result of this, there is a fixed relationship between the amplitude of the alternating voltage across the auxiliary winding and the constant voltage. The transformer in the known switched-mode power supply further has a secondary winding which is connected via a rectifier to a load to be powered. The turns ratio between the auxiliary winding and the secondary winding defines the amplitude of the alternating voltage across the secondary winding and, consequently, that of the rectified voltage across the load. In this way there is a fixed relationship between the direct voltage across the load to be powered and the constant voltage on the control input of the control circuit of the switcher IC. This known switched-mode power supply is very suitable for powering a load with a fixed voltage. The peak current through the switching transistor is limited by comparing the voltage drop across the switching transistor with a reference voltage and by forcing the switching transistor out of conduction as soon as the voltage drop exceeds the reference voltage. This peak current limitation protects the switching transistor against overloading and limits the power supplied by the power supply. However, if the load to be powered becomes so large as a result of, for example a short-circuit or if the load to be powered is a rechargeable battery which is still in a substantially exhausted condition, the a.c. component across the secondary winding decreases and, as a result of this, the a.c. component across the auxiliary winding also decreases. The rectified voltage across the smoothing capacitor, which also forms the supply voltage for the control circuit, then drops below a critical limit. Below this limit the control circuit is set to the so-called auto-restart mode, in which the control circuit attempts to build up enough voltage, with a duty cycle of 5 percent, across the load to be powered and, consequently, also across the smoothing capacitor to eventually allow a return to normal operation. Under these conditions the known power supply circuit does supply some power to the load to be powered but this is usually much lower than the desired power. If the load to be powered is a substantially exhausted rechargeable battery it will take a long time before the battery voltage, if in fact it does become high enough when the battery is full, has increased sufficiently to restore the known power supply circuit to normal operation and to subsequently charge the battery to a fully charged condition at a satisfactory rate.